Tides and long-term modulations in the Caribbean Sea

The tidal signal and its long-term variation in the Caribbean Sea is analyzed on the basis of hourly records from thirteen tide gauges five of which span more than 20 years. The seven larger tidal constituents are studied, namely the fortnightly term M-f, diurnal K-1, O-1, and P-1, and semidiurnal M-2, S-2 and N-2. The 18.61 year nodal modulation is clearly identifiable in almost all the examined constituents of lunar origin. However, its signal in M-2 is less clear while it is almost imperceptible in N-2, where the 8.85 year cycle caused by the eccentricity of the Moon's orbit and the orientation of its major axis variation dominates the long-term variability. The effect of the nodal variation in the amplitude and phase lag of the various tidal constituents is in agreement (within the 95% error limits) with the theoretical gravitational estimate, with the exception of the 8.85 year cycle in N-2, where larger values are found. Overall, in the Caribbean the net effect of the low frequency cycles can change the maximum tidal range from 16.5% to 23.5% in a nodal cycle. Although the Caribbean is a micro-tidal environment this still results in changes of the range of up to 8.4 cm. Significant, spatially coherent trends are found for the amplitude of S-2 (2.3 to 8.8 mm/cy).